
#include <cstddef>
using std::size_t;
#include "screen5.h"

//private class for use by ScreenPtr only
class ScrPtr {
    friend class ScreenPtr;
    Screen *sp;
    size_t use;
    ScrPtr(Screen *p): sp(p), use(1) { }
    ~ScrPtr() { delete sp; }
};

//相当于对Screen类的智能指针,ScrPtr即为对Screen的智能指针
class ScreenPtr {
public:
    // no default constructor: ScreenPtrs must be bound to an object
    ScreenPtr(Screen *p): ptr(new ScrPtr(p)) { }

    // copy members and increment the use count
    ScreenPtr(const ScreenPtr &orig):
       ptr(orig.ptr) { ++ptr->use; }
    ScreenPtr& operator=(const ScreenPtr&);

    // if use count goes to zero, delete the ScrPtr object
    ~ScreenPtr() { if (--ptr->use == 0) delete ptr; } 

    // constructor and copy control members as before
    Screen &operator*() { return *ptr->sp; }
    Screen *operator->() { return ptr->sp; }

    const Screen &operator*() const { return *ptr->sp; }
    const Screen *operator->() const { return ptr->sp; }
private:
    ScrPtr *ptr;        // points to use-counted ScrPtr class
};

ScreenPtr& ScreenPtr::operator=(const ScreenPtr &rhs)
{
    ++rhs.ptr->use;     // increment use count on rhs first
    if (--ptr->use == 0)
         delete ptr;    // if use count goes to 0 on this object, delete it
    ptr = rhs.ptr;      // copy the ScrPtr object
    return *this;
}

